1. Field of the Invention
The present invention relates generally to a control system for a vehicle power train; in particular, it pertains to a temperature control system whereby the transmission fluid in a hydraulically controlled automatic transmission can be prevented from being overheated in a severe thermal load condition of the engine and/or transmission.
2. Description of the Relate Art
A hydraulically controlled automatic transmission is known for years and disclosed, for instance, in "SERVICE MANUAL FOR RN3F01A-TYPE AUTOMATIC TRANSAXLE" issued by Nissan Motor Co., Ltd., the assignee of this invention. In this type of automatic transmission, an upshifting or downshifting gear change can be automatically achieved by selecting a speed range in accordance with a throttle pressure and a governor pressure both derived from a line pressure, which are supplied to shift valves in a hydraulic control circuit. The gear change can also be achieved manually, by a driver's operation of a control lever for selecting a speed range of the transmission.
In the case of electronically controlled automatic transmission, an upshifting or downshifting gear change from one speed range to another speed range is achieved by selectively fastening various friction elements such as clutches, brakes and so on, in accordance with electronic commands. The shift control uses a gear shift map which is based on various parameters, such as vehicle speed and throttle opening, and an upshifting or downshifting gear change is performed when the driving condition of the vehicle changes and the relevant parameter crosses a shift line in the shift map.
The hydraulically controlled automatic transmission can be constructed only from a hydraulic system, by replacing the electronic commands of the electronically controlled automatic transmission with hydraulic commands and performing a similar shift control using the throttle pressure and governor pressure. Thus, the hydraulically controlled automatic transmission can be manufactured at a relatively low cost in comparison to the electronically controlled transmission, and serves to achieve a significant cost reduction.
However, because the hydraulically controlled automatic transmission is not provided with an electronic control system, it may be difficult to adopt effective countermeasures to overheating of the mission fluid. Thus, for example, the hydraulically controlled type automatic transmission cannot perform a control operation for avoiding an overheated condition of the transmission fluid which may be caused by a continued driving of the vehicle under a severe thermal load condition of the engine and/or transmission.